JP2010222174A - Nitride semiconductor structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor structure having a flat plane or a hetero interface in an atomic level. <P>SOLUTION: The nitride semiconductor substrate 101 is GaN having (0001) plane as a principal orientation plane and 1×10<SP>5</SP>cm<SP>-2</SP>density of threading dislocations 104 including a helical component. A masking material 102 (a silicon oxide thin film having 100 nm thickness) is formed on the principal orientation plane of the nitride semiconductor substrate 101. The masking material 102 has a plurality of apertures 103, and each aperture 103 is a regular hexagon having 8 μm sides. A GaN thin film 105 having 500 nm thickness is formed in each aperture 103. By examining the relationship between the density of threading dislocations 104 and the area of the aperture 103 under various conditions, it is found that when the density of threading dislocations is N cm<SP>-2</SP>, the surface of the nitride semiconductor thin film formed in each aperture 103 is made flat provided that the area of each aperture 103 is not more than 1/N cm<SP>2</SP>. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor structure, and more particularly to a nitride semiconductor thin film structure having a flat surface or interface at an atomic level.

A nitride semiconductor is a compound of at least one element among group III elements such as B, Al, Ga, and In and nitrogen that is a group V element, and has a general formula of Al _1-abc B _{It is represented by a} Ga _b In _c N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ c ≦ 1). The flatness of the surface of the nitride semiconductor thin film or the interface of the heterostructure in which two or more nitride semiconductor thin films are laminated is desirably smooth at the atomic level from the viewpoint of elucidation of physical properties and device application. In particular, if the interface between the barrier layer and the quantum well layer is flat at the atomic level, the energy spread (fluctuation) of quantum levels (subbands) formed in the quantum well is reduced. As a result, the emission spectrum of the quantum well can be sharpened, and the characteristics of elements (resonant tunnel diodes, optical switch elements, cascade lasers, etc.) that use subbands can be improved.

  Here, in the epitaxial growth of GaAs which is an arsenide semiconductor, it has been reported that a flat surface or heterointerface can be obtained at the atomic level by a selective growth method in which a growth region is limited by a mesa or a mask material (Patent Document 1). And 2). In these inventions, the object is achieved by selectively growing in a region whose diameter is the shorter of the migration length of the thin film material grown on the semiconductor substrate or the lateral diffusion length in the gas phase. ing.

  However, in the epitaxial growth of nitride semiconductors, threading dislocations having a helical component extending from the semiconductor substrate and the interface between the semiconductor substrate and the epitaxially grown nitride semiconductor thin film are present in a high density. The flatness of the interface was impaired. In Non-Patent Document 1, because of the spiral growth originating from dislocations containing a helical component, the surface of the selectively grown GaN is composed of high-density atomic steps and terraces, and the flatness is worse than that of GaAs. Has been reported. The proportion of the largest atomic terrace in the growth region is at most 1-2%.

  Patent Documents 1 and 2 do not mention the influence of dislocations in the semiconductor substrate on the growth of the semiconductor thin film.

Japanese Patent Application No. 8-54422 Japanese Patent Application No. 8-153618

T. Akasaka, T. Nishida, S. Ando, and N. Kobayashi, Japanese Journal of Applied Physics vol. 37 (1998) pp. L842-844.

  As described above, in the nitride semiconductor thin film, even if the selective growth method is used, it is impossible to produce a flat surface or heterointerface at the atomic level due to the influence of threading dislocations having a helical component. Furthermore, the influence of the density of threading dislocations having a helical component and the size of the opening of the mask material on the flatness of the surface of the nitride semiconductor thin film and the heterointerface has not been studied.

  The present invention has been made in view of such problems, and an object thereof is to provide a nitride semiconductor structure having a flat surface or heterointerface at the atomic level.

  In order to achieve such an object, according to a first aspect of the present invention, there is provided a nitride semiconductor substrate, a mask layer formed on the nitride semiconductor substrate and having a plurality of openings, and the plurality of openings. A nitride semiconductor structure comprising a plurality of nitride semiconductor thin films formed on each of the portions, wherein the area of each opening is narrower than the reciprocal of the threading dislocation density including the helical component of the nitride semiconductor substrate. Features.

In addition, according to a second aspect of the present invention, in the first aspect, the threading dislocation density is 10 ³ cm ⁻² or more and 10 ⁷ cm ⁻² or less, and the nitride semiconductor thin film formed in each opening Among the atomic terraces constituting the surface, the area of the largest atomic terrace is 80% or more of the surface area.

  According to a third aspect of the present invention, there is provided a nitride semiconductor substrate, a mask layer having a plurality of openings formed on the nitride semiconductor substrate, and a plurality of openings formed in each of the plurality of openings. A nitride semiconductor structure including the nitride semiconductor multilayer thin film, wherein an area of each opening is narrower than a reciprocal of a threading dislocation density including a helical component of the nitride semiconductor substrate.

According to a fourth aspect of the present invention, in the third aspect, the threading dislocation density is 10 ³ cm ⁻² or more and 10 ⁷ cm ⁻² or less, and the nitride semiconductor multilayer thin film formed in each opening In the at least one interface, the widest atomic terrace among the atomic terraces constituting the interface is 80% or more of the area of the interface.

  According to a fifth aspect of the present invention, in any one of the first to fourth aspects, the nitride semiconductor substrate includes sapphire and a second mask material formed on a main orientation plane of the sapphire. And a nitride semiconductor thin film grown laterally on the second mask material.

  According to a sixth aspect of the present invention, in any one of the first to fourth aspects, the nitride semiconductor substrate includes silicon and a second mask material formed on a main orientation plane of the silicon. And a nitride semiconductor thin film grown laterally on the second mask material.

  According to a seventh aspect of the present invention, in any one of the first to fourth aspects, the nitride semiconductor substrate includes silicon carbide and a second mask formed on a main orientation plane of the silicon carbide. And a nitride semiconductor thin film grown laterally on the second mask material.

  According to an eighth aspect of the present invention, in any one of the first to seventh aspects, the opening is polygonal or circular.

According to the present invention, when the threading dislocation density including the helical component present in the substrate is N cm ⁻² , the area of each opening of the mask material is reduced to 1 / N cm ² or less. The surface of the nitride semiconductor thin film or the interface of the nitride semiconductor multilayer thin film formed in the above can be flattened.

It is a figure which shows the nitride semiconductor structure of Embodiment 1, (a) is a top view, (b) is a figure which shows sectional drawing. 2 is an atomic force microscope (AFM) image of one surface of a GaN thin film 105. FIG. It is a figure which shows the nitride semiconductor structure of Embodiment 2, (a) is a top view, (b) is a figure which shows sectional drawing. It is a figure which shows the nitride semiconductor structure of Embodiment 3, (a) is a top view, (b) is a figure which shows sectional drawing.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Although description will be made using examples in which specific numerical values such as density and thickness are specified, there is no intention to limit to these examples.

(Embodiment 1)
1A and 1B show the nitride semiconductor structure of the first embodiment. FIG. 1A represents a plan view, and FIG. 1B represents a cross section taken along the line AA ′ in FIG. The nitride semiconductor substrate 101 was GaN having a (0001) plane as a main orientation plane, and the density of threading dislocations 104 containing a helical component was 1 × 10 ⁵ cm ⁻² . A mask material 102 (silicon oxide thin film, thickness 100 nm) is formed on the main orientation surface of the nitride semiconductor substrate 101. A plurality of openings 103 are opened in the mask material 102. Each of the openings 103 is a regular hexagon having one side of 8 microns, and each side is parallel to the [11-20] direction of GaN. Furthermore, a GaN thin film 105 having a thickness of 500 nm is formed in each opening 103.

  FIG. 2 shows an atomic force microscope (AFM) image of one surface of the GaN thin film 105. Although several atomic steps are observed in the peripheral portion, it was found that the surface of the GaN thin film 105 was covered by 80% or more with a single atomic terrace. That is, the area of the largest atomic terrace among the atomic terraces constituting the surface of the nitride semiconductor thin film formed in each opening is 80% or more of the surface area of the nitride semiconductor thin film.

Here, since the density of threading dislocations 104 including the helical component of the GaN substrate that is the nitride semiconductor substrate 101 is 1 × 10 ⁵ cm ⁻² , the in-plane spacing of the threading dislocations 104 is about 30 microns. Therefore, a situation is realized in which no threading dislocation 104 is present in the majority of the openings 103 among the plurality of openings 103 each having a regular hexagonal shape with a side of 8 microns. For this reason, spiral growth that impairs the surface flatness in the conventional selective growth of GaN does not occur, and a flat GaN surface can be formed at the atomic level almost covered with a single atomic terrace.

Examining the relationship between the density of threading dislocations 104 and the area of the opening 103 under various conditions, when the threading dislocation density including a helical component is N cm ⁻² , the area of each opening 103 of the mask material 102 is It has been found that the surface of the nitride semiconductor thin film formed in each opening 103 becomes flat if it is 1 / N cm ² or less. When the value of N is 10 ³ or more and 10 ⁷ or less, the area of the largest atomic terrace among the atomic terraces constituting the surface of the nitride semiconductor thin film 105 formed in each opening 103 is the surface area. It was found to be 80% or more.

This relationship is not limited to the case where the nitride semiconductor substrate 101 or the nitride semiconductor thin film 105 is GaN, but also a nitride semiconductor Al _{1 -abc} B _a Ga _b In _c N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ c ≦ 1). However, a combination of compositions and a thin film thickness that do not cause misfit dislocations between the nitride semiconductor substrate 101 and the nitride semiconductor thin film 105 are required. Further, the main orientation plane of the nitride semiconductor substrate 101 is not necessarily (0001), and {1-100}, {1-101}, {11-20}, {11-21}, {11-22 } It may be a surface. Further, the shape of the opening 103 is not necessarily a regular hexagon, and may be another polygon or a circle.

(Embodiment 2)
3A and 3B show the nitride semiconductor structure of the second embodiment. 3A represents a plan view, and FIG. 3B represents a cross section taken along the line AA ′ of FIG. 3A. The nitride semiconductor substrate 101 is GaN having a (11-20) plane as a main orientation plane, and the density of threading dislocations 104 containing a helical component is 1 × 10 ⁵ cm ⁻² . A mask material 102 (silicon nitride thin film, thickness 50 nm) is formed on the main orientation plane of nitride semiconductor substrate 101. A plurality of openings 103 are opened in the mask material 102. The opening 103 is a square having a side of 20 microns, and each side is parallel to the [0001] or [1-100] direction of GaN. Further, the opening 103 has an n-type GaN layer 106a having a thickness of 500 nm, an undoped AlN layer 106b having a thickness of 3 nm, an undoped GaN layer 106c having a thickness of 2 nm, an undoped AlN layer 106d having a thickness of 3 nm, and an n-type having a thickness of 300 nm. A type GaN layer 106e is sequentially formed to form a nitride semiconductor multilayer thin film 106. When the cross section of the nitride semiconductor multilayer thin film 106 was observed with a transmission electron microscope, the area of the widest atomic terrace was 80% or more of the area of each interface at all interfaces. Therefore, it can be said that at least one interface of the nitride semiconductor multilayer thin film 106 formed in each opening 103 has an area of the widest atomic terrace constituting the interface of 80% or more of the interface area.

Examining the relationship between the density of threading dislocations 104 and the area of the opening 103 under various conditions, when the threading dislocation density including a helical component is N cm ⁻² , the area of each opening 103 of the mask material 102 is It has been found that the interface of the nitride semiconductor multilayer thin film 106 formed in each opening 103 becomes flat if it is 1 / N cm ² or less. When the value of N is not less than 10 ^{3 and not} more than 10 ⁷ , the largest atomic terrace among the atomic terraces constituting at least one interface of the nitride semiconductor multilayer thin film 106 formed in each opening 103 is formed. It was found that the area was 80% or more of the interface area.

Incidentally, this relationship is a nitride semiconductor substrate 101 and the nitride semiconductor multilayer film 106, a nitride having any composition semiconductor _{Al 1-a-b-c} B a Ga b In c N (0 ≦ a ≦ 1,0 ≦ b ≦ 1, 0 ≦ c ≦ 1). However, a combination of compositions that does not cause misfit dislocations between the nitride semiconductor substrate 101, the nitride semiconductor multilayer thin film 106, and each layer in the nitride semiconductor multilayer thin film 106, and the thickness of each layer are required. Further, the main orientation plane of the nitride semiconductor substrate 101 is not necessarily {11-20}, and (0001), {1-100}, {1-101}, {11-21}, {11-22] } It may be a surface. Furthermore, the shape of the opening 103 is not necessarily a square, and may be another polygon or a circle.

  The emission spectrum from the nitride semiconductor multilayer thin film 106 was measured at a temperature of 4K by the cathodoluminescence method. Compared with the conventional nitride semiconductor multilayer thin film in which the interface is formed with a high-density atomic step, the width of the emission spectrum is reduced to about one third. This is because the film thickness fluctuation of the 2 nm-thick undoped GaN layer 106c serving as the quantum well is reduced, and the energy spread (fluctuation) of the subband is reduced.

  In addition, ohmic electrodes were formed on the front surface of the nitride semiconductor multilayer thin film 106 and the back surface of the nitride semiconductor substrate 101, respectively, and resonant tunneling diodes were manufactured. When the current-voltage characteristics were measured at room temperature, the peak-to-valley ratio was 3 to 5 times that of a resonant tunneling diode fabricated from a conventional nitride semiconductor multilayer thin film whose interface was formed with high-density atomic steps. Could get. This is also because the film thickness fluctuation of the undoped GaN layer 106c having a thickness of 2 nm is reduced, and the energy spread (fluctuation) of the subband is reduced.

(Embodiment 3)
In the first and second embodiments, an example in which a GaN thin film on a sapphire substrate is used as the nitride semiconductor substrate 101 has been described. However, the nitride semiconductor structure of the third embodiment is illustrated in FIGS. Show. FIG. 4A represents a plan view, and FIG. 4B represents a cross section taken along the line AA ′ in FIG. The nitride semiconductor substrate 101 in this embodiment is composed of sapphire 101a having a (0001) plane as a main orientation plane, a second mask material 101b, and a laterally grown GaN thin film 101c. FIG. 4A shows the orientation of sapphire 101a. The second mask material 101b is a silicon oxide thin film having a thickness of 100 nm, and a large number of stripes parallel to the [1-100] direction of the sapphire 101a are formed. The stripe width can be 1 to 100 microns and the stripe spacing can be 1 to 100 microns. A mask material 102 and an opening 103 are formed on the surface of the laterally grown GaN thin film 101c. At this time, the opening 103 must be formed in a portion of the GaN thin film grown in the lateral direction on the second mask material 101b (see FIG. 4B). This is because the density of threading dislocations including the helical component of the portion of the GaN thin film grown in the lateral direction on the mask material 101b is small. As in the first and second embodiments, the opening 103 has a nitride semiconductor thin film 105 in which the area of the largest atomic terrace among the atomic terraces constituting the surface occupies 80% or more of the surface area, or at least one interface. The nitride semiconductor multilayer thin film 106 in which the area of the widest atomic terrace constituting the interface was 80% or more of the interface area could be formed.

Here, when the threading dislocation density including the helical component of the portion of the GaN thin film 101c laterally grown on the second mask material 101b is N cm ⁻² , the area of the opening 103 of the mask material 102 is 1 / N cm ² or less is required. This relationship laterally grown GaN thin film 101c, the nitride semiconductor thin film 105, the nitride semiconductor multilayer film 106, a nitride having any composition semiconductor _{Al 1-a-b-c} B a Ga b In c N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ c ≦ 1). However, a combination of compositions that do not cause misfit dislocations between the laterally grown GaN thin film 101c and the nitride semiconductor thin film 105 or the nitride semiconductor multilayer thin film 106, and between each layer in the nitride semiconductor multilayer thin film 106, Thickness is required.

  Further, silicon or silicon carbide may be used instead of sapphire 101a. In the case of sapphire or silicon carbide, main orientation planes other than (0001), such as {11-20}, {1-100}, {1-101}, {11-21}, {11-22} planes, etc. It may be used. In the case of silicon, principal orientation planes such as {111}, {100}, {110} can be used.

101 nitride semiconductor substrate 101a sapphire 101b second mask material 101c laterally grown GaN thin film 102 mask material 103 opening 104 of mask material threading dislocation 105 containing helical component nitride semiconductor thin film 106 nitride semiconductor multilayer thin film 106a n-type GaN layer 106b undoped AlN layer 106c undoped GaN layer 106d undoped AlN layer 106e n-type GaN layer

Claims (8)

A nitride semiconductor substrate;
A mask layer having a plurality of openings formed on the nitride semiconductor substrate;
A nitride semiconductor structure comprising a plurality of nitride semiconductor thin films formed in each of the plurality of openings,
The area of each opening is narrower than the reciprocal of the threading dislocation density including the helical component of the nitride semiconductor substrate. The threading dislocation density is 10 ³ cm ⁻² or more and 10 ⁷ cm ⁻² or less,
2. The nitride semiconductor structure according to claim 1, wherein the area of the widest atomic terrace among the atomic terraces constituting the surface of the nitride semiconductor thin film formed in each opening is 80% or more of the surface area. . A nitride semiconductor substrate;
A mask layer having a plurality of openings formed on the nitride semiconductor substrate;
A nitride semiconductor structure comprising a plurality of nitride semiconductor multilayer thin films formed in each of the plurality of openings,
The area of each opening is narrower than the reciprocal of the threading dislocation density including the helical component of the nitride semiconductor substrate. The threading dislocation density is 10 ³ cm ⁻² or more and 10 ⁷ cm ⁻² or less,
In at least one interface of the nitride semiconductor multilayer thin film formed in each opening, the area of the widest atomic terrace among atomic terraces constituting the interface is 80% or more of the area of the interface The nitride semiconductor structure according to claim 3. The nitride semiconductor substrate is
With sapphire,
A second mask material formed on the main azimuth plane of the sapphire;
The nitride semiconductor structure according to claim 1, further comprising a nitride semiconductor thin film grown laterally on the second mask material. The nitride semiconductor substrate is
With silicon,
A second mask material formed on the main orientation plane of the silicon;
5. The nitride semiconductor structure according to claim 1, further comprising a nitride semiconductor thin film grown laterally on the second mask material. 6. The nitride semiconductor substrate is
Silicon carbide,
A second mask material formed on the main orientation surface of the silicon carbide;
The nitride semiconductor structure according to claim 1, further comprising a nitride semiconductor thin film grown laterally on the second mask material.   The nitride semiconductor structure according to claim 1, wherein the opening is polygonal or circular.